Flotation is a known process for the separation of particulate materials from slurries or suspensions in a liquid, usually water. The particles which it is desired to remove from the suspension are treated with reagents to render them hydrophobic or water repellent, and a gas, usually air, is admitted to the suspension in the form of small bubbles. The hydrophobic particles come into contact with the bubbles and adhere to them, rising with them to the surface of the liquid to form a froth. The froth containing the floated particles is then removed as the concentrate or product, while any hydrophilic particles are left behind in the liquid phase and pass out as the tailings. The flotation process can be applied to suspensions of minerals in water, and also to the removal of oil droplets or emulsified oil particles, as well as to fibrous or vegetable matter such as paper fibres and bacterial cells and the like.
In most applications it is necessary to add reagents known as collectors which selectively render one or more of the species of suspended particles hydrophobic, thereby assisting in the process of collision and collection by the air bubbles. It is also usual to add frothing agents to assist in the formation of a stable froth on the surface of the liquid. The process of admitting these various reagents to the system is known as conditioning.
In conventional known cells, the contact between the air and the conditioned slurry is effected in a rectangular cell or tank having substantially vertical walls, the contents of the cell being stirred by a mechanical agitator which usually serves the additional purpose of breaking up the supply of air into small bubbles. In another known process described as column flotation, the conditioned suspension is introduced toward the top of a tall vertical column, and air bubbles are formed in the bottom of the column by blowing pressurized air through a diffuser. A layer of froth bearing the floatable particles forms above the liquid and overflows from the top of the column. The liquid containing the non-floating particles discharges from the bottom of the column. The position of the froth-liquid interface is maintained at a desired level by controlling for example the flow of liquid from the bottom of the column.
In some embodiments, wash water is introduced near the top of the froth layer to create a downflow of liquid which tends to reduce the entrainment of undesired gangue particles in the froth overflow.
In such known flotation columns, the liquid flows downward while the bubbles rise vertically upward. Since the rise velocity of the bubbles is related strongly to their size, the bubbles must be above a certain critical diameter in order that they may rise through the liquid and into the froth layer.
This method of operation using counter-current flow of liquid and bubbles possesses several operating difficulties or deficiencies when implemented. Any bubbles smaller than the critical size will be swept down the column and out in the tailings stream, carrying with them any floatable particles which may be adhering to them. Furthermore the necessity to operate with relatively large bubbles, typically in the range 1 to 3 mm in diameter, places a limit on the area of gas-liquid interface that can be created in the column. Since the quantity of particles that can be recovered from the bubbles, it would obviously be desirable to disperse the given quantity of air provided into the finest practicable size in order to give a large surface area and hence maximize the recovery of the particles.
Another disadvantage with known columns is that the proportion of bubbles in the total volume of the liquid phase in the column is relatively low, being typically in the range 10 to 20 percent. Thus the distance between bubbles is relatively large and the probability of contact between particles and bubbles is relatively lower than if the bubbles were very closely packed. A low probability of contact leads to low recovery rates of floatable particles, and to the necessity for very tall columns or a multiplicity of columns to achieve a desired yield.
A further disadvantage is related to the necessity in flotation columns to introduce the air through a diffuser made of porous material containing very fine holes. Such diffusers tend to block or become plugged, not only with fine particles but also from deposits which form by precipitation, especially when the liquid has a high concentration of dissolved solids.
It is the purpose of the present invention to provide a simple, efficient and economic means of conducting the flotation process which overcomes the difficulties inherent in known columns, by creating a stable dispersion of bubbles in the liquid, which bubbles may be as fine as desired without detriment to the process, and which may be present in very high void fractions thereby creating an environment highly favorable to the capture of the floatable particles.